1. Field of the Invention
The present invention relates to the use of carbollide systems for cation removal from aqueous media using electrochemical ion exchange. More particularly, it relates to novel cobalt carbollide materials useful in electrochemical ion exchange cells.
An exemplary electrochemical ion exchange (xe2x80x9cEIXxe2x80x9d) cell comprises two working electrodes positioned on either side of a chamber, each having an unflattened expanded mesh counter electrode in close proximity to its surface. The working electrode surfaces are exposed to, for example, a waste stream. FIG. 1 shows such an EIX flow cell. In FIG. 1, each working electrode 1 is embedded in an ion exchange material 2 (typically a resin) and has a mesh counter electrode 3 juxtaposed to its surface. The arrows a and b show where the waste stream enters and leaves the cell.
In use. therefore, an aqueous medium is passed through the chamber. A negative potential is applied to the weak acid cation exchanger causing rapid adsorption of ions to the cathode (working electrode) from the solution. The electric field across the ion exchange material layer between the current feeder and counter electrode encourages this cation migration.
For eluting the cations, a flush solution is passed through the chamber. Upon polarity reversal, the adsorbed ions can then be eluted into a limited volume of water to give a concentrated product. Elution is caused by the reversed electric field oxidising water at the current feeder of the working electrode, and thereby generating protons which displace the cations. The adsorption capacity of EIX electrodes can be used many times over under external electrical control.
Electrochemical ion exchange has been found to be a robust and effective process for active liquid wastes treatment, and to be capable of achieving high decontamination, volume reduction factors and a low energy consumption.
2. Brief Description of the Related Art
An EIX system has previously been developed by Turner et al. which allows ionic material to be adsorbed and eluted electrically by polarity reversal with great efficiency. Decontamination of a factor of 2000 has been observed for caesium removal with up to 75% loading of ion exchanger at flow rates of 8 bed volumes per hour. Unfortunately. the complexing agent used was not mentioned by Turner et al. but the context in which it was discussed would suggest it was an organic ion exchanger. Inorganic ion exchangers have also shown selectivity for cations.
Separated electrochemical cells have also been developed for removing ions such as strontium and caesium from aqueous solution. The cell consists of two compartments, each containing one electrode and separated by an anion selective membrane. Both electrodes may be working electrodes, or one may be acting as a counter electrode.
Platinized titanium has been successfully used as the working electrode due to its corrosion resistance. However, this material catalyses hydrogen gas production. Cheaper materials of unknown identity which are in development are claimed to be five times more cost effective.
Compounds consisting entirely of boron and hydrogen are termed boranes. Boranes exist as cages which can be closo, nido, arachno, or hypho. Closo is a fully closed cage, nido has the most electron deficient boron removed, and arachno has the two most electron deficient borons removed.
Boranes containing carbon are termed either carbaboranes or carboranes. Carboranes which have undergone degradation are usually referred to as carbollide ions. Ortho-carborane (7,8-dicarborane, C2B10H12) can be used to prepare the ortho-dicarbollide ion (C2B9H11), also referred to as 7,8-dicarbollide. This ion can easily be stored as nido-dicarborane (C2B9H12). The carbollides of many metals are known. These metals include first row transition metals such as iron and nickel, for example, and also f-block metals.
Cobalt Bis-7,8-dicarbollide (CDC)
Structure: [(C2B9H11)2Co]xe2x88x92. Registry Number: 11078-84-5.
IUPAC Name: Cobaltate(1-), bis [(7,8,9,10,11-.eta)-undecahydro-7,8-dicarba-undecaborato(2-)]-(9CI). 
CDC is a carborane product comprising cobalt (III) as a metal atom centre surrounded by two hemisphere cages of carboranes. The carborane ligands are of the nido structure variety.
CDC has been researched in detail in relation to caesium and strontium removal from aqueous solution. Since chlorinated cobalt bisdicarbollide is hydrophobic by nature, it allows the extraction of caesium and strontium ions by solvent extraction techniques. This process mainly concerns the use of nitrobenzene, which is toxic, and polyethylene glycol. As of August 1996, 26 cubic metres of high level waste has been reprocessed at the Mayak Production Association (PA) at Chelyabinsk2 using chlorinated CDC. The recovery degree for both strontium and caesium is over 99%. This work is being carried out by a joint U.S.-Russian research and development project at the Khlopin Radium Institute in St. Petersburg3.
A paper has been published on the attachment of cobalt bisdicarbollide to polymer resins12 using butyl lithium with the polymers polystyrene and polybenzimidazole. There is also a paper1 which describes putting organic chains on to the cobalt bisdicarbollide in an attempt to cause polymerisation by condensation reactions. The products were proposed for use in a liquid-liquid solvent extraction system. and also as the active sites of cation exchange on grafted polymer supports.
5,6,10-Hexachloro Cobalt bis-7,8-dicarbollide
Structure: [(C2B9H8Cl3)2Co]xe2x88x92. Registry Number: 107105-38-4.
IUPAC Name: Cobaltate(1-), bis[(7,8,9,10,11-eta.)-trichlorooctahydro-7,8-dicarba-undecaborata(2-)] (9CI). 
This hexachloro CDC derivative has been used in the solvent extraction of caesium into nitrobenzene at the Khlopin Institute as discussed earlier, as well as in the separation and detection of different chloro derivatives of cobalt dicarbollide by isotachophoretic determination6.
The chlorinated cobalt bisdicarbollide can be prepared by several methods7. The hexachlorinated CDC can be prepared by:
Cl2+THF/2-propanol+[Co(7,8-C2B9H11)2]xe2x88x92xe2x88x92uv/xcex3xe2x86x92[Co(5,6,10-Cl3, 7,8-C2B9H8)2].
Three patent documents discuss chlorinated CDC derivatives used in a solvent extraction system5.
Cobalt Dicarbollide Trimer
Structure: [Co3(C2B9H11)2(C2B8H10)2]3xe2x88x92. Registry Number: 59200-84-9.
IUPAC Name: Cobaltate(3-), bis[.mu.-[.eta.5-decahydrodicarbadecaborato(4-)]]bis [7,8,9,10,11-.eta.)-undecahydro-7,8-dicarbaundecaborato(2-)]tri- (9CI). 
In metal carbollide chemistry, the term xe2x80x9ctrimerxe2x80x9d refers to a complex containing 3 metal atoms (Co (III) in the illustrated case) in a multilayer xe2x80x9csandwichxe2x80x9d of four carbollide ions. Alternatively, such a trimer may be referred to as a trinuclear complex.
The trinuclear CDC complex is known only as a research curiosity. A recent paper by Volkov et al10 describes the production of several oligomers (polynuclear complexes). Earlier publications relate to the preparation and spectra characterisation of the trimer8 and crystallographic characterisation9 of its anion.
EP-A-0 150 602 discloses metal carbollides and their use as charge transfer mediators in enzyme based electrochemical assay systems.
WO-A-96/331132 discloses processes for extracting cesium and strontium ions from nuclear waste using substituted metal dicarbollides as extraction agents.
The present invention provides the use as an EIX ion exchange material in a flow through electrochemical cell of a metal carbollide and especially of cobalt carbollides. The invention also provides electrochemical ion exchange cells characterised in that the ion exchange material is a metal carbollide. Preferably the EIX cell comprises:
a housing having a chamber defined therein;
a working electrode disposed within the chamber and associated with the metal carbollide ion exchange material; and
a counter electrode juxtaposed to an exposed surface of the ion exchange material.
The metal carbollide normally contains a dicarbollide and more preferably the carbollide ions are exclusively dicarbollide ions. The dicarbollide is usually ortho-dicarbollide.
Another product of the invention is a metal carbollide which comprises a carbollide cage substituted by an organic moiety selected from carboxylic acids and thiols. The carbollide cage may be substituted by a halogen, especially chlorine, which provides a product of good stability. Alternatively, the carbollide cage may be substituted by an organic moiety having a functional group. The functional group may be a group which can be used to bond the carbollide to a metal (especially a working electrode) or a functionality useful for grafting the carbollide onto a polymer. A further class of substituents are those functional groups useful for interconnecting carbollide cages, suitably to form a polymer. More generally, substituents may be selected, for example empirically, to modify the properties of the polynuclear carbollide. Further aspects and embodiments of the invention will be apparent from the following description and claims.